RFC 5288 AES-GCM Cipher suites August 20081. Introduction
This document describes the use of AES [AES] in Galois Counter Mode
(GCM) [GCM] (AES-GCM) with various key exchange mechanisms as a
cipher suite for TLS. AES-GCM is an authenticated encryption with
associated data (AEAD) cipher (as defined in TLS 1.2 [RFC5246])
providing both confidentiality and data origin authentication. The
following sections define cipher suites based on RSA, DSA, and
Diffie-Hellman key exchanges; ECC-based (Elliptic Curve Cryptography)
cipher suites are defined in a separate document [RFC5289].
AES-GCM is not only efficient and secure, but hardware
implementations can achieve high speeds with low cost and low
latency, because the mode can be pipelined. Applications that
require high data throughput can benefit from these high-speed
implementations. AES-GCM has been specified as a mode that can be
used with IPsec ESP [RFC4106] and 802.1AE Media Access Control (MAC)
Security [IEEE8021AE].
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. AES-GCM Cipher Suites
The following cipher suites use the new authenticated encryption
modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]:
CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
These cipher suites use the AES-GCM authenticated encryption with
associated data (AEAD) algorithms AEAD_AES_128_GCM and
AEAD_AES_256_GCM described in [RFC5116]. Note that each of these
AEAD algorithms uses a 128-bit authentication tag with GCM (in
particular, as described in Section 3.5 of [RFC4366], the
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"truncated_hmac" extension does not have an effect on cipher suites
that do not use HMAC). The "nonce" SHALL be 12 bytes long consisting
of two parts as follows: (this is an example of a "partially
explicit" nonce; see Section 3.2.1 in [RFC5116]).
struct {
opaque salt[4];
opaque nonce_explicit[8];
} GCMNonce;
The salt is the "implicit" part of the nonce and is not sent in the
packet. Instead, the salt is generated as part of the handshake
process: it is either the client_write_IV (when the client is
sending) or the server_write_IV (when the server is sending). The
salt length (SecurityParameters.fixed_iv_length) is 4 octets.
The nonce_explicit is the "explicit" part of the nonce. It is chosen
by the sender and is carried in each TLS record in the
GenericAEADCipher.nonce_explicit field. The nonce_explicit length
(SecurityParameters.record_iv_length) is 8 octets.
Each value of the nonce_explicit MUST be distinct for each distinct
invocation of the GCM encrypt function for any fixed key. Failure to
meet this uniqueness requirement can significantly degrade security.
The nonce_explicit MAY be the 64-bit sequence number.
The RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon key exchanges
are performed as defined in [RFC5246].
The Pseudo Random Function (PRF) algorithms SHALL be as follows:
For cipher suites ending with _SHA256, the PRF is the TLS PRF
[RFC5246] with SHA-256 as the hash function.
For cipher suites ending with _SHA384, the PRF is the TLS PRF
[RFC5246] with SHA-384 as the hash function.
Implementations MUST send TLS Alert bad_record_mac for all types of
failures encountered in processing the AES-GCM algorithm.
4. TLS Versions
These cipher suites make use of the authenticated encryption with
additional data defined in TLS 1.2 [RFC5246]. They MUST NOT be
negotiated in older versions of TLS. Clients MUST NOT offer these
cipher suites if they do not offer TLS 1.2 or later. Servers that
select an earlier version of TLS MUST NOT select one of these cipher
suites. Because TLS has no way for the client to indicate that it
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supports TLS 1.2 but not earlier, a non-compliant server might
potentially negotiate TLS 1.1 or earlier and select one of the cipher
suites in this document. Clients MUST check the TLS version and
generate a fatal "illegal_parameter" alert if they detect an
incorrect version.
5. IANA Considerations
IANA has assigned the following values for the cipher suites defined
in this document:
CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
6. Security Considerations
The security considerations in [RFC5246] apply to this document as
well. The remainder of this section describes security
considerations specific to the cipher suites described in this
document.
6.1. Counter Reuse
AES-GCM security requires that the counter is never reused. The IV
construction in Section 3 is designed to prevent counter reuse.
Implementers should also understand the practical considerations of
IV handling outlined in Section 9 of [GCM].
6.2. Recommendations for Multiple Encryption Processors
If multiple cryptographic processors are in use by the sender, then
the sender MUST ensure that, for a particular key, each value of the
nonce_explicit used with that key is distinct. In this case, each
encryption processor SHOULD include, in the nonce_explicit, a fixed
value that is distinct for each processor. The recommended format is
nonce_explicit = FixedDistinct || Variable
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where the FixedDistinct field is distinct for each encryption
processor, but is fixed for a given processor, and the Variable field
is distinct for each distinct nonce used by a particular encryption
processor. When this method is used, the FixedDistinct fields used
by the different processors MUST have the same length.
In the terms of Figure 2 in [RFC5116], the Salt is the Fixed-Common
part of the nonce (it is fixed, and it is common across all
encryption processors), the FixedDistinct field exactly corresponds
to the Fixed-Distinct field, the Variable field corresponds to the
Counter field, and the explicit part exactly corresponds to the
nonce_explicit.
For clarity, we provide an example for TLS in which there are two
distinct encryption processors, each of which uses a one-byte
FixedDistinct field:
Salt = eedc68dc
FixedDistinct = 01 (for the first encryption processor)
FixedDistinct = 02 (for the second encryption processor)
The GCMnonces generated by the first encryption processor, and their
corresponding nonce_explicit, are:
GCMNonce nonce_explicit
------------------------ ----------------------------
eedc68dc0100000000000000 0100000000000000
eedc68dc0100000000000001 0100000000000001
eedc68dc0100000000000002 0100000000000002
...
The GCMnonces generated by the second encryption processor, and their
corresponding nonce_explicit, are
GCMNonce nonce_explicit
------------------------ ----------------------------
eedc68dc0200000000000000 0200000000000000
eedc68dc0200000000000001 0200000000000001
eedc68dc0200000000000002 0200000000000002
...
7. Acknowledgements
This document borrows heavily from [RFC5289]. The authors would like
to thank Alex Lam, Simon Josefsson, and Pasi Eronen for providing
useful comments during the review of this document.
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